1. Field of the Invention
This invention relates to the addition of a dispersant formulation which will effectively utilize water in gypsum wallboard manufacture to the end that the water quantity required in forming the slurry is reduced.
This invention also relates to the addition of a dispersant formulation which increases the diffusion rate of water from the wallboard during drying and, therefore, reduces the quantity of heat required to dry the wallboard.
This invention further relates to a process of adding a dispersant formulation to the slurry in wallboard manufacture which will reduce the quantities of water and drying heat required without sacrifice of the strength and flexibility of the product.
2. The Prior Art
There are several materials added for different purposes to gypsum stucco during the mixing of the slurry which is fed into the wallboard manufacturing process.
Starch, dispersant, accelerator, foam and water are some of the materials fed into the mixer with the stucco in order to make up the slurry for the wallboard formation. Other special materials are added where fire resistance and other characteristics are required of the wallboard. How these ingredients are added to the mixture is not of present concern.
What is important is that each ingredient has its own particular contribution to the mixture from which the final wallboard product is formed. For example, the starch is used to protect the gypsum crystals that form the bond with the paper. These crystals, being at the edges of the wallboard, are subject to dehydration during the period at high temperatures in the drying klins. If dehydration occurs, the paper will peel off the board core. The starch holds water during klin drying and protects these bonding crystals from severe dehydration. The accelerator is used as the catalyst to control the time that is desired to "set" the board on the production line. The production supervisor uses the minimum quantity of accelerator which gives him the hardening rate needed for the speed and equipment that makes up his production line. Too much accelerator can cause a weakened board core.
The dispersants are used to reduce the water carrying capacity of the stucco slurry. They also reduce surface tension and allow the slurry to flow with less frictional resistance. Foam is added in order to give about 25% by volume of soap bubbles to the slurry. This provides the board with the lighter weight necessary for easy handling, allows the board to be nailed without cracking or breaking, and gives some flexibility to the wallboard. Pulped paper is added to furnish a web of fibers to help hold the crystalline core together and also add flexibility to the finished product. Fiberglass has the same purpose in the fire rated wallboard products. Water is needed to make a slurry out of all the ingredients so that they will form a homogeneous mixture. The water also furnishes one ingredient needed for the reaction of stucco with water to make the gypsum wallboard. Therefore, water carries the ingredients and furnishes the medium in which the wallboard forming takes place. The ratio of water to stucco is usually 9 to 10. The slurry is discharged from the mixture onto a continuously moving paper covered line. This line normally runs at a speed of about 125 to 150 feet per minute. The paper is formed into an envelope which holds the stucco slurry until it has a chance to react and bond itself intimately to the paper. It then hardens, or sets, and then it can be cut and mechanically positioned for drying. Upon completion of the drying process, the board is bundled, taped and stacked for shipment. Due to the greater need for energy efficiency in manyfacturing processes, it is becoming important to cut down on the water required to manufacture gypsum wallboard, and thus, cut down on the energy required to dry the finished product.
Usually, 75% of the water added during forming must be evaporated out of the board during drying (about 1 pound per square foot of wallboard). This drying time is usually 45 to 60 minutes and requires 2 to 2.5 million BTU per one thousand square feet of regular 1/2 inch thick wallboard.
In order to make the process more efficient, it has become necessary to modify some of the standard additives. For example, reformulation of the dispersant is required in order to make it stronger acting so that the amount of water required may be reduced, while overcoming the properties causing soft, crumbly board and lengthened setting and drying time.